1. Field of the Invention
The present invention relates to a polyphase voltage transformer which magnetically cancels harmonic currents caused by non-linear loads, and thereby reduces the harmonic current percentage present in the input line power distribution network.
2. Description of Related Art
Harmonic currents present in a power distribution network can present significant problems, including power losses, overheating, resonances and overvoltages, operational instability, and radio frequency disturbances. Any electronic circuit which presents a non-linear load to the power source will inherently generate harmonic currents. In many applications, for example at data processing and telecommunication sites, power distribution feeders may carry up to 80% harmonic currents. Power thyristors, rectifiers, and "switch mode" power supplies commonly used in data processing and telecommunications equipment are inherently non-linear and are a major cause of power supply degradation due to generation of harmonics.
Numerous systems have been proposed for on-site harmonic current reduction, including proposals involving use of multiple transformers or multi-phase transformers with delta coupled secondary windings. An example of the former, involving use of two single phase transformers having primary and secondary windings coupled in quadrature, together with "filtering" transformers between each of the taps, the filter transformers having a low impedance to desirable "phase equal" currents, is disclosed in U.S. Pat. No. 3,671,901. An example of the latter is disclosed in U.S. Pat. No. 4,779,189, which describes a system in which harmonics are essentially reduced by greatly increasing the number of phases at the output.
All transformer systems involve some type of phase shifting in the secondary windings (a concept which dates back to the earliest "zig-zag" transformers), whether intentional or unintentional, but attempts to reduce harmonics have generally focused on using auxiliary circuitry to reduce the harmonics so that none are induced in the core. One prior attempt at on-site harmonic current reduction which used phase control is disclosed in U.S. Pat. Nos. 4,513,240 and 4,513,243. These patents disclose the addition of an auxiliary wye-connected secondary winding to the principal secondary windings (and in addition the use of dynamic stabilizers) to provide reactive power compensation by causing all three phases to be present in each combination of principal secondary windings and auxiliary secondary windings, the combination of phase rotation and positive leakage reactants in the system resulting in the selective cancellation of resonant currents produced in the dynamic stabilizers so that the harmonic currents are not present in the principal secondary windings, and therefore are not induced in the transformer core. While apparently effective, this type of system is best suited for application to heavy machinery, such as in locomotives, because of its size and complexity.
In general, prior solutions to the harmonics problem have proved to be impractical for non-industrial applications because they are either overly complex and therefore costly, excessively bulky, or inefficient. In the case of a transformer with delta connected secondary windings, for example, a 30.degree. phase shift can be used to delete some lower order harmonics, but this configuration can only be used to power very specific types of three phase loads. Since the delta connected secondary does not have a natural neutral, single phase loads and three-phase delta loads present problems. Most of the other known systems for reducing harmonics have similarly limited application.
In contrast, the present invention provides a harmonic suppression transformer in which all significant harmonic currents may be completely cancelled. This is accomplished using multiple wye-connected three-phase secondary outputs, and by phase shifting the outputs in such a way that the harmonics magnetically cancel each other in the transformer core and/or primary coils. As a result, a variety of load types may be accommodated.
While the technique of providing multiple three-phase wye connected outputs has previously been proposed, the technique has heretofore not been applied in a way which would suppress harmonics, Instead, the technique has been used for providing multiple outputs phase shifted in respect to each other by 180.degree., as disclosed in U.S. Pat. No. 2,470,598. The 180.degree. phase shift arrangement does not have the effect of cancelling harmonics.
Because it has previously been thought necessary to provide delta connected or complex auxiliary secondary windings for harmonic suppression purposes, with or without the addition of additional tuning or filtering means such as the filtering transformers disclosed in U.S. Pat. No. 3,671,901, cited above, or to provide stabilizers in connection with the connected secondaries as in U.S. Pat. No. 3,671,901, also cited above, those skilled in the art have heretofore been unable to take advantage of the simplicity and compatibility advantages of a wye connection in applications requiring harmonic current suppression without complex stabilizer and/or auxiliary winding systems.